LONDON — Wounded Londoners painted a nightmarish picture of chaos and bloodshed as four explosions ripped through the heart of their city on Thursday, leaving Britain in shock.

Injured and traumatized victims stumbled from Underground rail stations and from the mangled hulk of a bus, all targets of what the government called terrorist attacks.

Scores of people were wounded. Police said there had been three blasts on different parts of the Underground and one on a red double-decker bus near Russell Square, an area popular with tourists and close to the British Museum.

“I came out of the bank and walked 20 meters and I saw the bus, then it just blew up. The ground vibrated with the explosion, and the bus flew into the air. It was shattered. Everyone was running for their lives crying out,” said Ayobami Bello, a 42-year-old security guard.

“The scene afterwards was horrible, pieces of body on the ground. I saw three bodies on the ground and three just hanging out of the bus.”

The explosion ripped the bus open like a flimsy tin can.

“I was on the bus. I looked round and the seats behind me were gone,” said one disoriented woman.

A score of ambulances lined up to ferry the injured to hospitals.

Cell phone networks quickly became jammed with callers trying to reach loved ones, and shops filled with people begging to use phones.

Calm amid chaos
Loyita Worley, who was on an Underground “tube” train in the financial district, described the attack there: “There was just a big bang and the lights went out, then ash started falling and there was the smell of burning in the carriage. Some people panicked, mostly people kept calm.

“We heard some people shouting for help. I didn’t know what it was. I was wondering whether it was a fire.”

One man’s clothes had been blown off and he was totally black with soot, she said, but passengers remained calm even as debris fell down onto the roof of the carriage.

Christine O’Connor was on a Circle Line train west of the city center.

“I was in the last carriage, we pulled out of the station and then almost immediately there was an explosion; it just went very quiet. The carriage filled up with smoke,” she said.

“It was very well organized," said Sean Barron, 20, who helped treat the wounded there. "We took down everybody’s details and made a priority list as to who was the worst wounded."

Trapped like sardines
Angelo Power, a barrister emerging from the station, said a loud explosion rocked his train as it left King’s Cross.

There were flashes of light, people were screaming and smoke began to billow into the carriage, stuck in one of the Underground’s narrow and barely lit tunnels.

“People were covered in black soot and smoke," said Gary Lewis, 32, who was evacuated from a subway train at the King’s Cross station. "People were running everywhere and screaming. It was chaos. I came out into the ticket hall and saw casualties everywhere as medics tended to them.

“The one haunting image was someone whose face was totally black and pouring with blood.”

There was mayhem as 20 to 30 minutes passed before police reached the train to take survivors to safety. Women passed out and agitation turned to hysteria as people smashed the glass windows to escape billowing smoke but feared jumping out on to what they thought could be live electric lines.

Abrupt end to celebration
Where just a day before Londoners had celebrated winning the right to host the 2012 Summer Olympics, the city fell strangely silent, with public transport shut down and areas cordoned off as a well-rehearsed emergency plan swung into action. The mangled wreckage of the red bus sat at Russell Square, near the British Museum.

Many shops and offices closed early, and commuters congregated in bars and cafes to exchange news or simply seek company.

Reliant on buses and trains, few knew how or when they would get home.

Londoners, long used to bombs by Irish republican guerrillas, have enjoyed years of relative freedom from attacks despite the ever-present threat, but police had warned that a terrorist strike was inevitable.

Thursday’s events brought that threat sharply to the fore. Sirens, helicopters and footsteps were the sounds of a subdued city.

At the request of Queen Elizabeth II, the Union Jack flag flying over Buckingham Palace was lowered to half staff in a mark of respect for the dead and injured.